Wave filter

ABSTRACT

534,603. Impedance networks; transformers. STANDARD TELEPHONES &amp; CABLES, Ltd. March 29, 1940, No. 5685. Convention date, June 2, 1939. [Class 40 (iii)] [Also in Group XXXV] In a filter of the balanced type comprising two or more sections 5, 6, Fig. 1, in tandem, the capacity unbalance is reduced by the connection of one or more condensers in series or diagonal between the input and output terminals. The stray capacities may be represented by a lattice network, Fig. 2, which may be so balanced that an E.M.F. applied across the input terminals will produce no output current if the product of the series capacities is made equal to the product of the diagonal capacities. This is done by the addition of a single condenser CA, CB or CC, Fig. 1, of suitable value. To balance for longitudinal currents, however, it is necessary either to make all the capacities of the lattice equal, in which case all three condensers CA, CB, CC are required, or to make the capacities such that the unbalanced voltage to ground at the input end is equal to but 180 degrees out of phase with that at the output end, which result may be obtained by the use of only two condensers CA, CB or CA, CC. In the two-section filter shown in Fig. 3, having a differential transformer T1, T2, at each end, the transmission through the piezoelectric crystals X1, X2 is balanced against that through the condensers C1, C2 for all frequencies outside the pass band, the balance being improved by the additional condensers CA, CB as in Fig. 1. The transformers T1, T2 allow the filter to be connected between unbalanced circuits, and they are tuned to the mid-band frequency by the provision of shunt capacities C3- C6. In order to correct for inductive unbalance, the couplings between the single winding W3 and the balanced windings W1, W2 are adjustable simultaneously in opposite senses, and preferably in addition the coupling between W3 and W1, W2 in series is adjustable. In the construction shown in Fig. 4, the winding W3 is carried by a tube 21 which is angularly adjustable in a circular slot 28 in a slide 22, and the latter is movable in longitudinal grooves within a tube 20 on which the balanced windings W1, W2 are wound.

July 15, 1941. R, A SYKES 2,249,440

WAVE FILTER Filed June 2, 1959 A TTO/QNEV Patented July 15, 1941 WAVE FILTER RogerA. Sykes, Fanwood, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application June 2, 1939, Serial No. 276,981

20 Claims.

This invention relates to wave filters and more particularly to composite filters made up of more than one section.

The object of the invention is to increase the attenuation outside of the transmission band in a composite wave filter.

In a wave filter of the balanced type comprising two or more tandem sections the overall attenuation outside of the transmission band may be materially reduced by capacitive unbalance due to uncompensated stray capacitances and by inductive unbaiance due to the inequality of nominally equal inductances. In accordance with the present invention the capacitive unbalance is reduced by the addition of a properly chosen capacitor connected between an input terminal and either the corresponding output terminal or the diagonally opposite output terminal of the filter. A further improvement in the attenuation characeristic may, in general, be obtained by employing two added capacitors, one connected between an input terminal and the corresponding output terminal, and the other connected either between the remaining input and output terminals or between an input terminal and the diagonally opposite output terminal. The capacitors are preferably made adjustable to facilitate selection of the proper values.

Also in accordance with the invention the inductive unbalance is reduced by providing at each end of the filter a balanced three-winding transformer having an adjustment by means of which the coupling between the single winding and one of the two balanced windings may be increased while at the same time the coupling between the single winding and the other balanced winding is decreased. The transformer is preferably provided with a further adjustment for varying the coupling between the single winding and the two balanced windings in series so that the transformer may be adjusted to transmit the required band of frequencies. A suitable embodiment of the transformer is disclosed in which the two balanced windings are wound upon a hollow cylinder and the single winding is supported within the cylinder. Provision is made for adjusting the position of the inner coil longitudinally with respect to the outer windings and for adjusting the angle between the axes of the inner and the outer coils.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawing, of which- Fig. 1 is a schematic showing of a two-section wave filter with balancing capacitors in accordance with the invention;

Fig. 2 is an equivalent lattice network representing the stray capacitances effective between the input and output terminals of the filter of Fig. 1;

Fig. 3 shows the circuit of `a specific embodiment of the invention including balancing capacitors and end transformers with adjustments for balance;

Fig. 4 is a cross-section of a suitable three- Winding transformer having special adjustments; and

Fig. 5 is a perspective view of the sliding member and inner coil of the transformer of Fig. 4.

Fig. 1 shows diagrammatically a composite wave filter comprising a plurality of sections, such as 5 and 6, connected in tandem between a pair of input terminals i 2 and a pair of output terminals 3, 4. The individual sections may be of any type but it is assumed that the filter as viewed at either end is a balanced structure. The capacitors CA and CB, each connected between an input terminal and an output terminal, are added in accordance with the invention to balance the stray capacitances associated with the component filter elements and thereby increase the attenuation of the filter outside of the transmission band. For a perfect balance to a common ground at both ends of the filter a third capacitor, such as Co, is required. The capacitors are preferably made adjustable, as indicated by the arrows.

The considerations involved in choosing the location and magnitude of the balancing capacitors are best explained by reference to Fig. 2 which shows `the equivalent lattice network representing the stray capacitances effective between the terminals oi' the filter of Fig. l. The capacitances C13 and C24 forming the series branches of the lattice represent, respectively, the stray capacitances effective between each input terminal I, 2 and the corresponding output terminals 3, 4. The capacitances C14 and C23 in the diagonal branches represent, respectively, the stray capacitances effective between each input terminal l, 2 and the diagonally opposite output terminals 4, 3. In general, all of the four capacitances forming the lattice will have different values and there is thus provided, in effect, a second transmission path in parallel with the normal path through the filter sections.

In accordance with the invention the stray capacitances represented by the lattice of Fig. 2

may be so balanced by the addition of a single capacitor that an electromotive force impressed upon the input terminals l and 2 in Fig. 2 will produce no current in a load connected to the output terminals 3 `and 4. The relationship to be satisfied is that the product of the capacitances in the series branches must be equal to the product of the capacitances in the diagonal branches. In equation form,

It is apparent, therefore, that if the product of the series capacitances is smaller than the product of the diagonal capacitances, an added capacitor such es CA or Cc of Fig. l is connected in parallel with one of the series branches. On the other hand, if the product of the diagonal capacitances is smaller than the product of the series capacitances, the added capacitor is connected between either of the input terminals and the diagonally opposite output terminal, as illustrated in Fig. l by the capacitor CB. In either case the added capacitor is given such a value that Equation 1 is satisfied.

The equivalent lattice of Fig. 2 even when balanced by the addition of a single capacitor, as explained above, will still permit the iiow of longitudinal currents. That is to say, an electrornotive force effective between ground and the two input terminals will 'cause a current to flow to ground through a load impedance connected across the output terminals. This current may be eliminated by making all of the branches in the lattice equal. This can be done by the addition of three capacitors, such, for example, as CA, CB and Cc in Fig. 1. A capacitor is connected in parallel with each of the branches except the one having the largest stray capacitance and these capacitors are given the proper values to make all of the branches of the lattice equal.

The effects of the longitudinal current in the output load may also be eliminated by the addition of only two capacitors so proportioned and so placed that the unbalance voltage to ground at the input end of the lter is made equal in magnitude to, but 180 degrees out of phase with, the unbalance voltage to ground at the output end. To accomplish this, Equation 1 must be satised and in addition the two series capacitances must be made equal. The latter condition in equation form is The choice of the branches to which the capacitors are to be added is based upon the following considerations. Since the two series branches are to be equal it is seen from Equation 1 that each series capacitance must be equal to the geometric mean of the two diagonal capacitances. That is,

C12 C2i 1/014023 (3) Therefore, ii each series capacitance is smaller than the geometric mean of the diagonal capacitances, a capacitor is added to each series branch and its value is so chosen that the total capacitance of each series branch is equal to this mean. Under these conditions the capacitors CA and Cc of Fig. 1 are used. On the other hand, if either or both of the series capacitances are larger than the geometric mean of the diagonal capacitances, the smaller series capacitance is built out to the Value of the larger by the addition of a capacitor and either of the diagonal capacitances is built out by the addition of a second capacitor of such a value that the product of the series branches is equal to the product of the diagonal branches. In this case the capacitors CA and CB may, for example, be used.

Fig. 3 shows an embodiment of the invention in a two-section filter which includes balancing capacitors to reduce the capacitive unbalance and balanced three-winding transformers with special adjustments to reduce the inductive unbalance. 'I'he two sections have similar circuit configurations and are designed to pass the same band of frequencies. Each section is of the differential transformer type in which 'the transmission through a piezoelectric crystal impedance is balanced against the transmission through an impedance of different character, for example, a capacitor. The two impedances are so proportioned that substantially complete balance is obtained at all frequencies except those falling in the transmission band.

One section of the lter comprises a threewinding transformer T1, a piezoelectric crystal X1 and a balancing capacitor C1, and the other sec-tion includes the corresponding elements T2, X2 and C2. The two sections are connected in tandem back-to-back, with the transformers at the outer ends, so that the composite lter is symmetrical and is seen as a balanced structure at the input terminals l, 2 and also at the output terminals 4. As explained above the capacitors CA and CB are added to reduce the capacitive unbalance caused by the stray capacitances shown in Fig. 2 which are effective between the various lter termonals.

On the assumptions that the stray capacitance C13 is larger than C24 and also larger than the geometric mean of C14 and C23, the capacitance CA is connected between terminals 2 and 4 and is given a value equal to .the difference between C13 and C24. The capacitance CB is now added to a diagonal branch, for example, between terminals l and 4, as shown. and given such a value that the product of the series branch capacitances is equal to the product of the diagonal branch capacitances. Alternatively, the capacitance Cn may be connected between terminals 2 and 3. Under other circumstances the capacitance CA may be connected between terminals I and 3, and CB may be connected either between terminals l and 4, as shown, or between terminals 2 and 3. On the other hand, if each stray series capacitance is smaller than the geometric mean of the diagonal capacitances, the capacitance CA may be connected as shown between terminals 2 and 4 and CB connected between terminals I and 3, and their values so chosen that the total capacitance of each series branch is equal to this mean. Also, as explained above, considerable improvement may be obtained by the use of a single balancing capacitor.

In the input transformer Ti the two balanced windings W1 and W2 are equal in inductance and are closely coupled. The third inductance W3 is inductiveiy coupled to the balanced windings. The two variable capacitors C3 and C4 are connected in shunt at the respective ends of the transformer and adjusted to resonate with the associated induetances at the mid-band frequency of the filter in order to decrease the transmission loss introduced by the transformer. In order to reduce the inductive unbalance in the lter due to an inequality in the windings W1, W2 or to other causes, an adjustment is provided by means of which the coupling between W3 and Wi can be increased or decreased while at the same time the coupling between W3 and W2 is decreased or increased. A further adjustment is provided to' vary the coupling between W3 and the windings W1 and W2 in series so that the proper coupling can be provided to make the transformer a bandpass network. The output transformer T2 is of similar construction and comprises the windings W4, Ws and W and the capacitors C5 and C6.

If the filter is to be used between balanced circuits and the adjustment for inductive balance is not required, the loads may be connected directly to the input terminals I, 2 and the output terminals 3, 4, in which case the windings W3, We and the associated tuning capacitors C3, C5 may be omitted. When these elements are included, however, the loads are connected to the input terminals II, I2 and the output terminals. I3, I4 and the filter may be used in either balanced or unbalanced circuits. By a proper adjustment of the diiferential coupling between the single windying and the balanced windings the inductive unbalance may be substantially eliminated and the attenuation of the filter outside of the transmission band materially increased. The addition of balancing capacitors and the proper adjustment of the differential coupling have increased the attenuation by as much as 30 decibels in actual cases.

A suitable structure for the three-winding transformer T1 is shown in cross-section in Fig. 4. The balanced windings W1 and W2 are wound on the tube 20, and the third winding W3 is placed on a smaller tube 2| which is supported within the tube 20 by the sliding member 22. As is shown more clearly in the perspective view of Fig. 5 the slide 22 has beveled edges 23, 2li, which t into the oppositely disposed longitudinal grooves 25, 26 in the inner surface of the tube 20. One end of the slide has a slot 21 leading to a circular hole 28 which has a diameter somewhat larger than the width of the slot. Each end of the tube 2I has a pair of notches 30, 3I which permit the tube to be slid through the slot 21 into the hole 28. The distance between diagonally opposite notches is equal to the diameter of the hole and therefore the tube is supported at at least three points, such as 33, 34 and 35.

By turning the tube 2I around in the hole the angle between the axis of the inner coil W3 and axis of the outer windings W1 and W2 can be adjusted, and therefore the inductive coupling between these coils can be set at the required value. Furthermore, by moving the slide 22 to the left or to the right the inner coil W3 may be displaced to one side or the other of a central position, and in this way the coupling between the single winding W2 and one of the balanced windings, say W1, may be increased while at the same time the coupling between W3 and the other balanced winding is decreased. This latter adjustment does not affect appreciably the coupling between W3 and the two balanced windings in series, as this is dependent primarily upon the angle between the axes of the coils.

What is claimed is:

1. A balanced wave filter comprising a pair of input terminals, a pair of output terminals, a plurality of sections connected in tandem between said pairs of terminals, and means for reducing the capacitive unbalance due to stray capacitance comprising an added capacitor connected between an input terminal and an output terminal whereby the attenuation of the filter outside of the transmission band is increased.

2. A wave filter in accordance with claim 1 in which said capacitor is connected between an input terminal and the corresponding output terminal.

3. A wave filter in accordance with claim 1 in which said capacitor is connected between an input terminal and a diagonally opposite output terminal.

4. A Wave filter in accordance with claim 1 in which said capacitor is adjustable.

5. A balanced wave filter comprising a pair of input terminals, a pair of output terminals, a plurality of sections connected in tandem between said pairs of terminals, and means for reducing the capacitive unbalance due to stray capacitance comprising a plurality of added capacitors, each of said capacitors being connected between an input terminal rand an output terminal, whereby the attenuation of the filter outside of the transmission band is increased.

6. A wave filter in accordance with claim 5 in which two of said capacitors are connected respectively between each of the input terminals and the corresponding output terminal.

'7. A wave filter in accordance with claim, 5 in which one of said capacitors is connected between an input terminal and the corresponding output terminal and another of said capacitors is connected between an input terminal and the diagonally opposite output terminal.

8. A Wave filter in accordance with claim 5 in which two of said capacitors are connected respectively between each of the input terminals and the corresponding output terminal and another of said capacitors is connected between an input terminal and the diagonally opposite output terminal.

9. A wave filter in accordance with claim 5 in which said capacitors are adjustable.

10. In a balanced wave iilter comprising a plurality `of sections connected in tandem between a pair of input terminals and a pair of output terminals, means for reducing the capacitive unbalance due to unequal stray capacitances effective between said terminals comrprising an added capacitor connected between an input terminal and an output terminal, the capacitance of said capacitor being so proportioned with respect to said stray capacitances that the product of the capacitances effective between each input terminal and the corresponding output terminal isA substantially equal to the product of the capacitances effective between each input terminal and the diagonally opposite output terminal.

11. In a balanced wave filter comprising a plurality of sections connected in tandem between a pair of input terminals and a pair of output terminals, means for reducing the capacitive unbalance due to unequal stray capacitances effective between said terminals comprising a plurality of added capaci-tors, each of said capacitors being connected between an input terminal and an output terminal, and the capacitances of said capacitors being so proportioned with respect to said stray capacitances 'that the capacitances effective between each input terminal and the corresponding output terminal are substantially equal and their product is substantially equal to the product of the capacitances effective between each input terminal and the diagonally opposite output terminal.

12. A balanced wave filter comprising means for reducing inductive unbalance including a three-winding transformer comprising a single winding and two substantially balanced windings, said transformer having an adjustment for increasing the coupling between said single winding and one of said balanced windings while at the same time decreasing the coupling between said single winding and the other of said balanced windings.

13. A wave lter in accordance with claim 12 in which said transformer has an additional adjustment for increasing or decreasing the coupling between said single winding and both of said balanced windings.

14. A balanced wave lter comprising a pair of input terminals, a pair of output terminals and a plurality of sections connected in tandem between said pairs of terminals, each end section of said lter including a three-winding transformer for connecting said filter with one of its terminal loads, each of said transformers comprising two substantially equal windings connected in series between one of said pairs of terminals, a third winding, and means for increasing the coupling between said third winding and one of said equal windings while decreasing the coupling between said third winding and the other of said equal windings.

15. A wave filter in accordance with claim 14 which includes a capacitor connected between an input terminal and an output terminal to reduce the capacitive unbalance.

16. A wave iilter in accordance with claim 14 which includes a plurality of added capacitors for reducing the capacitive unbalance, each of said capacitors being connected between an input terminal and an output terminal.

17. A Wave filter in accordance with claim 14 in which each of said transformers includes additional means for adjusting in the same direc- 'tion the coupling between said third winding and both of said equal windings.

18. A balanced wave lter comprising a pair of input terminals, a pair of output terminals, a plurality of sections connected in tandem between said terminals, and an added capacitor connected between an input terminal and an output terminal and adjusted to decrease the capacitive unbalance due to stray capacitance, each end section of sail lter including a threewinding transformer for connecting the lter to a terminal load and each of said transformers comprising a single winding and two balanced windings and including means for increasing the coupling between said third winding and one of said balanced windings while decreasing the coupling between said third winding and the other of said balanced windings.

19. A wave filter in accordance with claim 18 in which each of said transformers includes additional means for adjusting in the same direction the coupling between said third winding and both of said balanced windings.

20. A wave filter in accordance with claim 18 which includes a second added capacitor connected between an input terminal and an out put terminal and adjusted to further decrease the capacitive unbalance.

ROGER A. SYKES. 

